Closures made of metal or plastics are used to seal e.g. bottles containing mineral water, juices, carbonated soft drinks and water, beer and other liquid products. The sealing closures, such as a twist crown, a crown cork or a screw cap, for bottles, jars and the like, commonly have an elastic sealing element on their inside facing the container mouth, which sealing element is pressed or clamped between the closure and the container when the closure is fitted and ensures tightness.
While formerly the liners of crown corks or bottle screw closures were predominantly made of press cork which was, if appropriate, also coated with a thin plastics film or thin aluminium foil, today sealing elements are commonly produced completely from synthetic polymer compounds. The sealing element of closures such as crown corks, screw caps and screw lids, which are usually made of metal (such as aluminium) or plastics (such as polyolefine material), often takes the form of a liner adhered to, or placed loose in, the closure.
Such sealing elements or closure liners must meet several requirements, as e.g. good sealing properties, low closing and opening torques in the case of screw-on closures and good adhesion to the cap material to ensure good retention of gases such as carbon dioxide and closure integrity. Moreover, the closures should have a valve or venting effect in case of any excess pressures arising in the container, in particular in beer, lemonade or mineral water bottles, and said venting effect should reliably occur at certain internal pressures. It is also important that the sealing elements or liners are capable of withstanding headloads as encountered in packaging, storage and handling of e.g. filled bottles, without damage to the liner.
One of the most important demands on sealing elements or liners is in the barrier properties of the sealing elements or liners. Ideally, the sealing elements or liners should prevent any substances which would affect aroma or taste, from entering the container or bottle. Moreover, it is clear that the liner material should have no flavor of its own which it can impart to the contents of the container. Changes of the taste of the container's contents are mainly due to oxidative aroma changes after the entry of oxygen, and changes of taste caused by the entry of organic flavor-affecting compounds. The general class of compounds contributing to undesirable off-taste comprise in particular aromatic substances such as benzenes or also phenyl ethers, for instance chloroanisoles. The latter are e.g. contained in wood pallets and cardboard boxes used for the transport or storage of beverage containers.
Furthermore, the closures or the barrier properties of the sealing elements or liners should also prevent the migration of flavors or volatile flavoring substances from the inside of the container or the bottle. The escape of the volatile flavoring substances e.g. during storage of the filled containers causes a flat taste of the beverage, which is of course very undesirable.
Beverage manufacturers have long searched for improved barrier materials. The sealing elements or liner materials known in the art, having at least some barrier properties, typically comprise polyvinylchloride, polyethylene or mixtures of these substances with vinylacetate or vinylacetate-ethylene copolymers. Although the use of PVC or other halogen-containing plastics leads to difficulties in the disposal and neutralization of the used container, PVC is still used in practice. It is also known in the art to use oxygen-bonding substances (oxygen scavengers) for improving the barrier properties of closures. These substances are reducing agents, intended to react with oxygen in the sealing element so that the oxygen does not reach the container contents. However, oxygen scavengers often do not have perfect sensory properties and they lose their effect with time.
An important improvement in the barrier properties of container closures is disclosed in U.S. Pat. No. 5,731,053. According to U.S. Pat. No. 5,731,053 the barrier properties of container closures can be improved by using a sealing element or liner, which is formed by placing and in-shell or out-shell molding, a molten piece of a polymer liner compound in the closure, wherein the molded sealing element or liner comprises a heterogeneous blend of butyl rubber and thermoplastic polymer. In the blend, the thermoplastic polymer, which preferably is HDPE, forms a continuous phase in the solid molded state, wherein the butyl rubber is contained in discrete separate phases or areas. In other words, the polymer liner compound according to U.S. Pat. No. 5,731,053 provides a pseudolaminate structure, with areas or layers enriched in butyl rubber adjoining or even alternating with areas or layers enriched in thermoplastic polymer, said layers being discrete enough to form an overall heterogeneous structure, and generally a structure with two separate solid phases.
In WO 97/33044 the use of cyclodextrin in rigid or semi-rigid cellulosic sheets is disclosed. The cyclodextrin acts as a barrier or a trap for contaminants. The barrier properties of the material disclosed in WO 97/33044 are based on entrapment of the respective permeants in the internal hydrophobic space of the cyclodextrin molecule. The cyclodextrin material is generally used in the form of a compatible, derivatized cyclodextrin. According to WO 97/33044 the preferred cyclodextrin is a derivatized cyclodextrin having at least one substituent group bonded to the cyclodextrin molecule.